The present invention relates to the use of steam in pressurized vertical reactor vessels and steam explosion processes to pre-treat biomass feed stocks, such as agricultural residues (including stalks, stover and hulls), straws and grasses, forest, sawmill residues (including wood chips and shredded thinnings) and lignocellulosic material, collectively referred to as biomass.
Biomass is heated and pressurized with steam in a first reactor, such as a vertical reactor vessel. As the heated biomass moves from the vertical reactor vessel, the biomass moves through an expansion valve or other expansion device to cause the biomass to undergo a steam explosion process. An exemplary steam explosion process for pre-treatment of biomass is described in U.S. Pat. No. 8,057,639. A similar process for pulping of wood chips is disclosed in U.S. Patent Application Publication 2008/0277082.
Steam, besides being the source of heat and pressurization for the biomass in the vertical reactor vessel, is the propellant that moves the biomass from the vertical reactor vessel, through a discharge pipe (conduit) and towards the expansion device. The amount of steam, gas or vapor needed to propel the biomass may be significant, such as 50 to 500 kilograms of steam to bone-dry ton (kgs/BDT) of biomass. The amount of steam may be injected at an elevation corresponding to the lower region of the vertical reactor vessel and may be needed to propel the biomass is typically dependent on the process, rate and volume of biomass moving through the vertical reactor vessel, and the dimensions and conditions of the vertical reactor vessel and related equipment.
Horizontal reactor vessels effectively use steam to propel biomass from the horizontal reactor vessel towards a steam explosion device. Steam added to the inlet of a horizontal reactor vessel remains in the horizontal reactor vessel and above the biomass. While some of the steam condenses in the horizontal reactor vessel, some of the steam also remains as a gas applying pressure to the biomass along the entire length of the horizontal reactor vessel. Because the steam in a horizontal reactor vessel extends the length of the horizontal reactor vessel and is immediately above the biomass at the outlet end of the horizontal reactor vessel, the steam propels the biomass through the outlet of the horizontal reactor vessel without disrupting the flow of biomass in the horizontal reactor vessel.
Conventional vertical reactor vessels have a disadvantage as compared to horizontal reactor vessels with respect to steam explosion pulping. Steam is added to the inlet at the upper end of a vertical reactor vessel. The steam is added to heat the biomass, pressurize the vertical reactor vessel and propel the biomass out a bottom discharge of the vertical reactor vessel. To propel the biomass, the steam must pass down through the biomass in the vertical reactor vessel to reach the bottom outlet. A relatively large amount of extra steam is required to ensure that sufficient steam is still present to propel the biomass out of the conventional vertical reactor system. In a vertical reactor vessel, steam moving down through the biomass may form gas passages, e.g., a rat-holing effect, through the biomass. These passages may cause the biomass to experience uneven retention periods in the vertical reactor vessel and affect the quality of the process.
In some respects, vertical reactor vessels are more efficient than horizontal reactor vessels. Vertical reactor vessels more efficiently use their volume because biomass occupies a greater portion of a vertical reactor vessel than is occupied by biomass in a horizontal reactor vessel. Vertical reactor vessels typically may be built much larger than horizontal reactor vessels and, thus, have a greater capacity for biomass throughput than a horizontal reactor vessel.